In the practice of professional audio, setting up and maintaining multiple speakers in a sound distribution system, whether indoors or outdoors, presents recurring problems of determining whether the available audio power is distributed in a optimal manner amongst the numerous speakers. Individual speakers are rated for impedance in ohms, as measured at a specified audio frequency, e.g. 330 Hz or 1 kHz. Multiple speakers are commonly combined in a group connected in series, parallel or in a series-parallel network.,
In a systematic approach, the system may be fed through one or more audio transmission lines for which a standard line voltage is specified. Individual speakers and/or speaker groups may be connected in parallel across the line at various points along the line. A transformer may be used with each speaker, speaker group and/or at each speaker tapoff along the line to balance the power level in each speaker. Standard line voltage levels have been established, e.g. 70.7 volts for high powered outdoor public address systems and 25 volts for indoor speaker systems such as in public schools.
Typically the impedance of speakers is predominantly resistive at the low frequency end of the audio spectrum and predominantly reactive (inductive) at the high frequency end.
A sound professional working in this field is primarily concerned with power distribution and needs to be able to quickly determine how much power will reach a load unit connected across the voltage-rated line. Due to present unavailability of cost-effective test equipment particularly dedicated to this problem, such professionals have had to settle for the conventional practice of making measurements with an impedance meter and then performing the additional step of calculating power from each impedance measurement, using a hand calculator or other means.